A program aimed at synthesis of cytotoxic macrocyles and their heteroatombridged analogs is proposed. Synthetic targets have been chosen among known cytotoxic or antitumor agents and nitrogen or sulfur-containing analogs will be prepared by ring expansion methodology developed in our laboratory. In general, the electrophilic sites which are required for antitumor activity will be incorporated into early synthetic intermediates. Macrocycles of varying size and heteroatom functionality will then be attached, and the resulting compounds will be screened for cytotoxicity. Active compounds will be submitted for in vivo antitumor screening. Emphasis will be placed on preparation of heterocyclic precursors of the carbocyclic cytochalasins. The bicyclic AB skeleton which is a basic requirement for cytotoxicity is already available from the previous grant period. Methods are proposed for conversion of the AB fragment into N- or S-containing macrocycles having all of the functionality needed for cytotoxicity in ca. 10 steps. Synthesis of carbocyclic analogs of cytochalasin D and deoxaphomin will be undertaken. This will require development of general methods for heteroatom extrusion from the macrocycle. The methods proposed will be used for synthesis of analogs related to the maytansine and asperdiol antitumor agents. Potentially active, simplified derivatives of these compounds will be prepared using new, convergent variants of our ring expansion methodology.